U.S. patent application Ser. No. 659,567 filed Feb. 22, 1991, U.S. Pat. No. 5,136,017 (corresponding to Canadian Patent Application No. 2,056,549 filed Nov. 28, 1991) discloses a continuous process for the polymerization of lactide to polylactic acid. The patent does not contemplate a process beginning with lactic acid as a raw material. Furthermore, the patent application does not disclose any procedure by which lactide may be prepared from lactic acid. Nor does the patent application disclose a process starting from lactic acid which may be operated continuously.
WO 90/01521 in the name of Battelle Memorial Institute published Feb. 22, 1990 discusses the polymerization of polylactic acid while maintaining residual monomer in the polymer to act as a plasticizer. The disclosure is of interest in that there is a fairly lengthy discussion of the prior art processes used to prepare polylactic acid. Most of the processes described as prior art disclose polymerizing lactic acid or lactide. There is no strong discussion regrading the preparation of lactide.
European Patent Application 0 261 572 in the name of Boehringer Ingelheim KG published 16.09.87 discloses a process in which polylactic acid is heated in the presence of a catalyst, and under vacuum, at temperatures from 130.degree. to 230.degree. C. and lactide is distilled off. The polylactic acid should have a molecular weight from 400 to 2,000, preferably from 500-800. In the polymerization of lactic acid, lactic acid is first heated. Initially low molecular weight polylactic acid forms. However, as the reaction proceeds the low molecular weight polylactic acid begins to depolymerize and form lactide. Then an equilibrium is established between lactide, lactic acid, water and low molecular weight polylactic acid. The result is that it is very difficult to directly polymerize high molecular weight polylactic acid from lactic acid. Rather, one must first prepare lactide and then subject it to a ring opening polymerization. However, the above noted equilibrium makes it difficult to produce a relatively pure stream of lactide from an equilibrium mixture. Applicants have found that the above process can be integrated into a continuous process to produce high molecular weight polylactic acid. Additionally, applicants have found that by using the principle of fractional distillation, crude lactide can be separated from the equilibrium mixture. Furthermore, purification of crude lactide can be carried out by melt crystallization (i.e. recrystallization of molten crude lactide without the use of a solvent). This has not been suggested by any prior art.
There is a need for technology which:
(i) provides for the synthesis and separation of lactide from lactic acid; PA1 (ii) provides for the purification of crude lactide without the use of significant amounts of solvent; and PA1 (iii) provides in combination a continuous process for the polymerization of polylactic acid to high molecular weight polymer. PA1 (i) dehydrating aqueous lactic acid and polymerizing lactic acid by condensation to a molecular weight from 400 to 5,000; PA1 (ii) heating and reacting an equilibrium mixture comprising mainly polylactic acid having a molecular weight from 400 to 5,000, and small amounts of water, lactic acid, lactide and linear oligomers of lactic acid and lactide; depolymerizing polylactic acid to form lactide, PA1 (iii) distilling off a vapor phase comprising mainly lactide, lactic acid, linear oligomers of lactic acid and water; PA1 (iv) in a separate unit operation further fractionating the distillate from step (iii) into crude lactide and a distillate phase, comprising in addition to lactide, water, lactic acid, and linear oligomers, which is optionally recycled to step (i); PA1 (v) purifying the crude lactide by melt crystallization; and PA1 (vi) bulk or solution polymerizing the purified lactide.